1. Field of the Invention
The present invention relates to a shaft sealing device for the sliding portion of a needle valve stem for use in a spray gun for painting, particularly in an airless paint spray gun for spraying compressed paints. More specifically, the present invention relates to a shaft sealing device capable of enhancing the sealing effect by inserting an auxiliary packing made of an elastic rubber body into a main packing for a needle valve having its stem pierced through the central portion of the main packing, tightening the auxiliary packing to thereby tighten the main packing, and protecting the sliding portion of the needle valve stem from a compressed fluid.
2. Description of the Prior Art
There have heretofore been proposed various paint spray guns such as in U.S. Pat. No. 3,000,576, U.S. Pat. No. 3,116,020, and Japanese Patent Publication No. SHO 54(1979)-7822. However, none of these citations specifically disclose in detail sealing means for the sliding portion of a needle valve used in a spray gun. In view of the necessity to compare the prior art with the sealing device of the present invention, therefore, a typical conventional sealing means of this kind will be described hereinafter with reference to an enlarged cross-sectional view of the prior art shown in FIG. 1. The conventional sealing means comprises, as shown, a packing 10 having a substantially V-shaped cross section, a packing rest 12 inserted into the substantially V-shaped groove in the packing 10, a belleville spring 13 for urging the packing rest 12 toward the bottom of the groove, and a packing gland 14 for pushing the packing 10 from behind 10, tightening the packing and thereby sealing the sliding portion of a needle valve stem 15.
In the case of this sealing means, when the packing 10 is worn off in the amount of a in consequence of repeated sliding reciprocation of the needle valve stem 15 used for a long period of time, it is necessary to further tighten the packing by use of the packing gland 14 to thereby supplement the aforementioned amount a in order to obtain a tight seal. However, since the non-sliding portion b is not worn away, the packing gland 14 collides against the surface of the non-sliding portion and cannot further tighten the packing 10 and, as a result, a tight seal cannot be obtained. Thus, this sealing means has a drawback that it is inferior in durability.
In order to eliminate the aforementioned drawback with the construction described above, precise processing tolerance is required. In other words, slight processing errors greatly affect the durability of a sealing means. To be more specific, in a case where the packing 10 comes into collision with the packing rest 12 not at the inner circumferential surface d shown in FIG. 2(A) but at the outer circumferential surface c alone due to the processing errors, as illustrated in FIG. 2(A), the surface pressure p becomes large relative to the outer circumferential surface c and small relative to the inner circumferential surface d, with the result that there gives rise to an incomplete seal. Even when the packing 10 is tightened by the packing gland 14 until the outer circumference of the groove of the packing 10 is elastically deformed so that the inner surface may be sealed up, wear of the sliding portion causes the packing gland 14 to collide against the surface of the non-sliding portion as described above and consequently causes the packing gland 14 to be prevented from moving forward, with the result that a complete seal cannont be obtained.
In the case where the packing 10 has its leading end e coming into collision with the packing rest 12, as shown in FIG. 2(B), the surface pressure p becomes locally high. This brings about disadvantages that the needle vlave stem 15 is interrupted in its return stroke, that the local portion on which high surface pressure p is exerted is relatively rapidly worn off, and consequently that the durability becomes inferior.
Further, the belleville spring 13 is required to have its spring force large enough to endure the tightening force of the packing gland 14 relative to the packing 10 and, at the same time, a large tightening force is required in tightening the packing 10 against the large spring force. When the packing 10 is made of a material which is easily deformable, such as leather, temporarily complete seal can be obtained by the deformation of the packing material, whereas the packing material cannot maintain its suitable size and shape intact and consequently cannot be used for a long period of time. Particularly when packing leather is used as a packing material, it absorbs water and a solvent to become hardened.